This invention relates to vacuum booster devices of automotive brake master cylinder and, more particularly, to improvements in those of the type comprising a booster shell, a booster piston axially slidably accommodated in the booster shell, a piston diaphragm the inner and outer peripheral edges of which are respectively fixed to the rear surface of the booster piston and the inner peripheral surface of the booster shell, a front side first working chamber and a rear side second working chamber defined in the booster shell by the booster piston and piston diaphragm, said first working chamber communicating with a vacuum supply source, said second working chamber selectively communicating with the first working chamber or the external atmosphere through a control valve, an input rod arranged opposite to the booster piston for movement toward and away from the latter and connected to the control valve so as to produce a pressure difference between both the working chambers effective to cause the booster piston to follow forward movement of the input rod, tie rods extending through the booster piston for connection of the front and rear walls of the booster shell, and a sealing means arranged between the tie rod and booster piston for enabling the booster piston to be operable.
In case of the above-mentioned booster device, the booster shell can be protected from the effect of the forward thrust loaded from the output side by transmitting the load to the automobile body through the tie rods, so that such rigidity high enough to bear the above loading is not required to be given and, what is advantageous, the booster shell can be made less in weight by being formed of thin steel sheets, synthetic resins or the like.
When said piston diaphragm is provided with the through holes for passing the tie rods therethrough, the tensile rigidity of the piston diaphragm shows a lowering tendency and, as a result of increase in the elongation of the piston diaphragm due to the pressure difference between both the first and second working chambers, the thrust load due to the above pressure difference is not effectively applied to the booster piston on occasion during the boosting operation.